The invention relates to the field of microelectronics, and especially to the field of microsystem components. It relates more particularly to microcomponents of the variable microcapacitor or microswitch type which integrate a membrane that can deform under the action of electrostatic forces. It also relates to a particular process allowing such microcomponents to be obtained, which proves to be greatly advantageous over the existing processes.
It is known that the microcomponents of the microcapacitor or microswitch type have a fixed plate and a movable membrane which are separated by a volume whose dimensions can vary, especially for example when a continuous potential difference exists between the fixed plate and the deformable membrane.
By subjecting the fixed plate and the movable membrane to a particular potential difference, it is thus possible to vary the nominal value of the capacitance according to the desired application.
In certain situations, it is also possible to ensure that the movable membrane moves sufficiently close to the fixed plate to make contact. The component is then used as a microswitch.
Microcomponents of this type are generally obtained by using techniques associated with surface micromachining. Conventionally, such a membrane is obtained by processes carried out at high temperature, above 400xc2x0 C., and more generally above the temperatures that can be withstood by a finished semiconductor without any risk of its functionalities being too greatly modified.
A component typically consists of a stack of thin layers, with the following conventional structure. A first layer constitutes the fixed plate of the capacitor. This layer may be made of polysilicon of the first level when the technologies used are compatible with semiconductor processing methods. This polysilicon layer is deposited on an oxide or nitride layer. In the document Darrin J. Young and Bernhard E. Boser xe2x80x9cA Micromachine-Based RF Low-Noi se Voltage-Controlled Oscillatorxe2x80x9d, IEEE 1997 Custom Integrated Circuits Conference, May 1997, pp. 431-434, microcomponents were also described in which the fixed plate is made of aluminum.
The layer that has to provide the future volume between the fixed plate and the movable plate is made of a sacrificial material deposited on top of the layer forming the fixed plate by any process compatible with the type of microcomponent that it is desired to produce. Thus, this material may conventionally be silicon dioxide (SiO2) or a derivative compound which will be removed by acid etching.
The upper layer, forming the deformable plate of the microcomponent is conventionally made of polysilicon obtained by LPCVD (low-pressure chemical vapor deposition) and is doped sufficiently to reduce its resistivity. Certain techniques propose a conductive coating placed on top of the polysilicon in order to decrease the apparent resistivity thereof. This movable plate is anchored to the substrate through vias, that is to say holes etched in the oxide and/or the sacrificial film. This movable and deformable plate may also be slightly textured by features etched partially in the oxide or sacrificial film before the latter is deposited. This texturing gives the lower face of the movable plate a certain relief and limits the area of contact between the fixed plate and the movable plate in the case in which they touch. Thus, any bonding phenomena are avoided.
In order to dissolve the sacrificial layer by chemical etching, or any process allowing isotropic etching, it is essential for the layer constituting the movable plate to also be pierced over its entire area in the form of regular and repeated patterns allowing the dissolving solution to pass.
A major drawback of this type of process is that the deposition of the polysilicon layer requires the use of a high-temperature technique which is not compatible with deposition on semiconductor layers whose functionalities run the risk of being significantly modified or degraded by heat. It is therefore impossible with this kind of technique to produce microcomponents directly on existing integrated circuits by a xe2x80x9cpost-processingxe2x80x9d technique.
The invention therefore aims to overcome these various drawbacks.
The invention therefore relates to a process for fabricating electronic microcomponents of the variable capacitor or microswitch type, comprising a fixed plate and a deformable membrane which are located opposite each other.
This process comprises the following steps, consisting in:
depositing a first metal layer of complex shape on an oxide layer, said first metal layer being intended to form the fixed plate;
depositing a metal ribbon, forming a border, on at least part of the periphery and on each side of the fixed plate, said ribbon being intended to serve as a spacer between the fixed plate and the deformable membrane;
depositing a sacrificial resin layer over at least the area of said fixed plate;
generating, by lithography, a plurality of wells in the surface of said sacrificial resin layer;
depositing, by electrolysis, inside the wells formed in the sacrificial resin, at least one metal region intended to form the deformable membrane, this metal region extending between sections of the metal ribbon which are located on each side of said fixed plate;
removing the sacrificial resin layer.
In other words, the process according to the invention allows production of deformable membranes produced by electrolysis, which process is carried out at room temperature. This therefore allows the microcomponent to be placed on various substrates, including integrated circuits.
Thus, in a first family of applications, the process according to the invention may be implemented using, as oxide layer, a quartz layer in order to form components incorporating only microcapacitors or microswitches.
In another type of application, the process may be implemented using, as oxide layer, an oxide layer deposited on an integrated circuit so that the microcapacitors or microswitches may be placed directly on top of the integrated circuit and can interact with functional regions of the integrated circuit, thereby limiting as far as possible the influence of the connection system since these microcomponents are closer to the integrated circuit. High integrated density is also achieved.
In practice, the first metal layer intended to form the fixed plate of the microcomponent is advantageously inserted into a recess formed in the oxide layer. In other words, the fixed metal plates may be obtained by a xe2x80x9cdamascenexe2x80x9d metallization process. This allows particularly reliable components to be obtained, since they are strong and vibration-resistant. Furthermore, by virtue of the excellent flatness of the layers obtained by this process, it is possible to superpose several layers without building up topological irregularities. The subsequent operations are thus facilitated.
In practice, the first metal layer forming the fixed plate advantageously includes an extension associated with a connection pad mounted on or inside the oxide layer. This connection pad allows the microcomponent to be linked either to the subjacent integrated circuit or to other parts of an electronic circuit.
According to another characteristic of the invention, the spacer ribbon is present along the periphery of the fixed plate, on two opposed sides of the latter. The segments of this ribbon then serve to support the ends of the deformable membranes.
In practice, the spacer ribbon may consist of a continuous band, or even advantageously of a succession of individual segments, present only in the regions receiving the ends of the deformable membranes.
In practice, the sacrificial resin layer is advantageously deposited in such a way that it partly covers the peripheral spacer ribbon. In this way, when the deformable membrane is deposited on top of the sacrificial resin layer, the region where it joins the spacer ribbon has breaks in slope, facilitating flexure of the deformable membrane.
In practice, the process according to the invention advantageously also includes a step consisting in etching the oxide layer in order to form one or more anchoring grooves intended to accommodate part of the peripheral ribbon, or else part of the ends of the deformable membrane. This anchoring groove is located directly outside the peripheral ribbon, or else partly beneath the peripheral ribbon. This groove accommodates part of the peripheral ribbon or else part of the membrane in order to ensure that it is deeply anchored in the substrate, thereby increasing the robustness of the microcomponent.
In practice, it has been determined that the anchoring is satisfactory when the groove advantageously has a width about twice the thickness of the deformable membrane and that, complementarily, the depth of the groove is more than one and a half times its width.
According to another characteristic of the invention, the process may furthermore include a step consisting, after the fixed plate has been deposited, in depositing a film of dielectric, intended to prevent the deformable membrane from bonding to the fixed plate. Thus, direct contact between the fixed plate and the deformable plate, which could cause these two plates to bond together, and therefore damage the microcomponent, is avoided. The value of the capacitance per unit area may, furthermore, be improved if the dielectric constant of the additional film is greater than that of air.
According to another characteristic of the invention, it is possible to complete the process according to the invention by adding an additional step consisting, after the sacrificial resin has been removed, in producing, on the upper face of the deformable membrane or membranes, raised regions capable of modifying the moment of inertia of the surface of the deformable membrane so as to produce membranes with programmed deformation. This is because, as already mentioned, the modification in the capacitance of the microcomponent may arise from modification in the spacing of the deformable membrane with respect to the fixed membrane when the latter are subjected to a DC voltage component. It is therefore possible, by virtue of this arrangement, to adapt the deformation of the deformable membrane, and therefore the variation in the capacitance, to a variation in the DC component to which the microcomponent is subjected.
In practice, the raised features produced on the membranes may advantageously be longitudinal ribs.
In practice, the metals used to produce the various plates may advantageously be chosen from the group comprising, especially, copper, chromium, nickel and alloys including these metals. Different metals may be used to produce the plates and the spacer ribbon. The choice of the various materials and of the various electrolysis conditions makes it possible to accurately establish the internal stresses in the deformable membrane.
By virtue of this characteristic, it is possible to give the deformable membrane a cross section which varies over its length.